Macro‐ and microalgal extracts as functional feed additives in diets for zebrafish juveniles

The search for natural immunotherapeutic agents has become increasingly important for fish disease treatment and prophylaxis. The present research explored the effects of macro- and microalgal extracts on zebrafish juvenile growth and immune status. A plant-based diet was used as a control (diet C), and seven other diets were prepared similar to diet C, supplemented with 10 g Kg−1 of each algal extract (Fucus vesiculosus, F; Nannochloropsis gaditana, N; and Ulva rigida, U), or a combination of extracts (diets FN, FU, NU and FNU). A fishmeal-based diet was included as a positive control (diet C+). Diet C+ outperformed the remaining dietary treatments, growth-wise. None of the extracts improved diet C negative effects. However, the inclusion of U. rigida and N. gaditana extracts promoted an immunomodulatory action after 1 week of trial, by upregulating cytokine expression. This effect subsided after 30 days, suggesting tolerance may be developed over time. In contrast, effects on growth-related genes were still observed after 5 weeks of feeding F, N, U and FN diets. Algal extract dietary inclusion did not compromise intestinal integrity. These findings support the potential of algal extracts as functional feed additives during short periods (<30 days) to enhance immunomodulation.